Introduction
4 Introduction
fixing and ranging) channel, existing at 6001200 m
depth in mid-latitudes, through which sound travels
long distances without attenuation [1.5]. Distances traveled by sound waves satisfy the inverse-square law and
decrease with increase in frequency – low-frequency
waves may travel tens of thousands of kilometers without significant attenuation, while very high frequency
waves typically penetrate distances of order 1 m. Hydrophones measure pressure fluctuations induced by
sound in Pa (micro Pascal), and the amplitude or
the loudness of the underwater sound is measured in
decibel with reference to a standard pressure level at
a standard distance, written dB re 1 Pa at 1 m. As
a reference, blue whales vocalize at 1040 Hz at source
sound levels of 155188 dB re 1 Pa at 1 m [1.6, 7],
whereas large ships and fast-moving small boats can
produce broadband (201000 Hz) sound levels with
source levels of 150200 dB re 1 Pa at 1 m. An excellent description of underwater acoustics is provided
in a sister handbook [1.5].
Since seawater is a conducting fluid, with conductivity in the range 2:56 S=m, electric fields in the
ocean maybe generated through induction by time varying external fields or by motion of the seawater through
the Earth’s magnetic field. Strong temporal variations in
the magnetic fields in the ionosphere and the magnetosphere induce electric fields in the ocean and generate
secondary magnetic fields. Further, the dynamo effect
of currents in the ocean, involving motion of conducting seawater through the Earth’s magnetic field, induce
electric fields, and, in turn, give rise to secondary magnetic fields in the water column [1.8]. Whereas the
electromagnetic waves in air propagate at the speed
of light (3 10
8 m=s), in the ocean the speed depends
on the wave frequency. At 1 Hz, it is approximately
1600 m=s, several orders of magnitude lower than in
air, while at 10 kHz, it is 100 times faster. The waves
undergo transmission losses, the rate of attenuation
increasing with frequency; a 10 kHz electromagnetic
wave is attenuated at a rate over 80 times faster than
a 1 Hz wave. Finally, at frequencies of order 10
14 s
1 ,
with corresponding wavelengths of 400700 nm (in
air), is the visible part of the electromagnetic spectrum.
Light waves propagate at the speed of 2:24 10
8 m=s in
seawater compared with 3 10
8 m=s in air. The attenuation of light is wavelength-selective, leading underwater
objects having bluish or greenish tints [1.9]. Ocean
electromagnetics are discussed in Chap. 8.
Ocean signals are typically noisy and have to be
processed to discern the underlying signatures that enable detection, identification, and location of objects
underwater. Typical processing is in terms of spectral
analysis of time series of signals, using for example
Fast Fourier transforms. Sophisticated algorithms are
required to extract underlying signatures from excessively noisy signals. The science of signal processing
enables development of the algorithms that facilitate
such extraction. It is discussed in Chap. 9.
In the past decade, significant effort has gone into
shipboard automation and development of unmanned
ocean systems. Underlying this effort are major advances in microelectronics and computer technologies.
Unmanned systems typically involve sensors and actuators that may be linked via a computer that acquires
and stores data from the sensors, formulates a response
using onboard control algorithms and sends out an actuation control signal, in support of achieving a desired
state. The error between the outcome of the actuation,
determined via an appropriate sensor measurement, and
the desired state is continuously monitored in a feedback loop and used to improve the control signal in
an iterative process. A simple proportional-integralderivative (PID) controller is based on using the present
values of the error, accumulated past values of the error,
and predicted future values of the error in developing an
improved actuation control signal. The control theories
and case studies illustrating the theories are discussed
in Chap. 10.
1.4 Applications
Human activities offshore and in coastal regions span
across a range of sectors, including shipping and maritime transport, offshore energy, security and defense,
development of ports, harbors and other coastal structures, fisheries and aquaculture, recreational activities
and activities related to mitigating environmental impacts of these activities. Planning, design, and conduct
of these activities as well as development of enabling
technologies involve applications of ocean engineering at various levels through the maritime industry.
In Parts B through E, we cover four major areas of
applications of ocean engineering in the maritime domain: automated unmanned systems, coastal design and
structures, offshore platforms, and offshore renewable
energy.
1.4.1 Automated Unmanned Systems
Automated unmanned systems are systems that are
pre-programmed to carry out desired tasks. Taking advantage of the advances in electronic and computer
technologies, significant strides are being made in the
4 Introduction
fixing and ranging) channel, existing at 6001200 m
depth in mid-latitudes, through which sound travels
long distances without attenuation [1.5]. Distances traveled by sound waves satisfy the inverse-square law and
decrease with increase in frequency – low-frequency
waves may travel tens of thousands of kilometers without significant attenuation, while very high frequency
waves typically penetrate distances of order 1 m. Hydrophones measure pressure fluctuations induced by
sound in Pa (micro Pascal), and the amplitude or
the loudness of the underwater sound is measured in
decibel with reference to a standard pressure level at
a standard distance, written dB re 1 Pa at 1 m. As
a reference, blue whales vocalize at 1040 Hz at source
sound levels of 155188 dB re 1 Pa at 1 m [1.6, 7],
whereas large ships and fast-moving small boats can
produce broadband (201000 Hz) sound levels with
source levels of 150200 dB re 1 Pa at 1 m. An excellent description of underwater acoustics is provided
in a sister handbook [1.5].
Since seawater is a conducting fluid, with conductivity in the range 2:56 S=m, electric fields in the
ocean maybe generated through induction by time varying external fields or by motion of the seawater through
the Earth’s magnetic field. Strong temporal variations in
the magnetic fields in the ionosphere and the magnetosphere induce electric fields in the ocean and generate
secondary magnetic fields. Further, the dynamo effect
of currents in the ocean, involving motion of conducting seawater through the Earth’s magnetic field, induce
electric fields, and, in turn, give rise to secondary magnetic fields in the water column [1.8]. Whereas the
electromagnetic waves in air propagate at the speed
of light (3 10
8 m=s), in the ocean the speed depends
on the wave frequency. At 1 Hz, it is approximately
1600 m=s, several orders of magnitude lower than in
air, while at 10 kHz, it is 100 times faster. The waves
undergo transmission losses, the rate of attenuation
increasing with frequency; a 10 kHz electromagnetic
wave is attenuated at a rate over 80 times faster than
a 1 Hz wave. Finally, at frequencies of order 10
14 s
1 ,
with corresponding wavelengths of 400700 nm (in
air), is the visible part of the electromagnetic spectrum.
Light waves propagate at the speed of 2:24 10
8 m=s in
seawater compared with 3 10
8 m=s in air. The attenuation of light is wavelength-selective, leading underwater
objects having bluish or greenish tints [1.9]. Ocean
electromagnetics are discussed in Chap. 8.
Ocean signals are typically noisy and have to be
processed to discern the underlying signatures that enable detection, identification, and location of objects
underwater. Typical processing is in terms of spectral
analysis of time series of signals, using for example
Fast Fourier transforms. Sophisticated algorithms are
required to extract underlying signatures from excessively noisy signals. The science of signal processing
enables development of the algorithms that facilitate
such extraction. It is discussed in Chap. 9.
In the past decade, significant effort has gone into
shipboard automation and development of unmanned
ocean systems. Underlying this effort are major advances in microelectronics and computer technologies.
Unmanned systems typically involve sensors and actuators that may be linked via a computer that acquires
and stores data from the sensors, formulates a response
using onboard control algorithms and sends out an actuation control signal, in support of achieving a desired
state. The error between the outcome of the actuation,
determined via an appropriate sensor measurement, and
the desired state is continuously monitored in a feedback loop and used to improve the control signal in
an iterative process. A simple proportional-integralderivative (PID) controller is based on using the present
values of the error, accumulated past values of the error,
and predicted future values of the error in developing an
improved actuation control signal. The control theories
and case studies illustrating the theories are discussed
in Chap. 10.
1.4 Applications
Human activities offshore and in coastal regions span
across a range of sectors, including shipping and maritime transport, offshore energy, security and defense,
development of ports, harbors and other coastal structures, fisheries and aquaculture, recreational activities
and activities related to mitigating environmental impacts of these activities. Planning, design, and conduct
of these activities as well as development of enabling
technologies involve applications of ocean engineering at various levels through the maritime industry.
In Parts B through E, we cover four major areas of
applications of ocean engineering in the maritime domain: automated unmanned systems, coastal design and
structures, offshore platforms, and offshore renewable
energy.
1.4.1 Automated Unmanned Systems
Automated unmanned systems are systems that are
pre-programmed to carry out desired tasks. Taking advantage of the advances in electronic and computer
technologies, significant strides are being made in the